I've just bought an Onoto piston filler at an antique shop. It's a BHR pen much like the top illustration in Marshall and Oldfield, 2nd ed., p.166, except that there is only an underfeed.
The plug can be unscrewed and the rod pulled out although I've not tried to see how far it can be extracted with force. The section unscrews nicely from the barrel assembly, revealing the pointed end of the piston. I suspect that the pen has not been serviced in a long time.
Having read M+O’s description a full overhaul is well beyond my current and likely future capabilities, In any case, the mechanism will always be fragile, which does not mix well with my innate clumsiness.
As the section does unscrew so easily, I can envisage removing the section and using the pen as an eyedropper by dropping ink into the barrel with the section out, then refitting the section with silicone grease. This would, of course, depend on the rod/barrel seal being sound, but I’d expect to test that out after greasing the rod and working it back and forth a few times. I’d also expect to have to release the ink flow by opening the knob a touch (if I understand the mechanism correctly).
So, what do Onoto experts see as the likelihood of being able to use the pen in this way, and are there any disadvantages, for example the possibility of damage or difficulty for future repairers due to ink getting into the wrong places?